There is provided a catalytic method for converting nitrogen oxides to nitrogen (i.e., N.sub.2). The catalyst for this method comprises an acidic solid comprising a Group IVB metal oxide modified with an oxyanion of a Group VIB metal. This method may be used for reducing emissions of nitrogen oxides from waste gases, including industrial exhaust gases and automobile exhaust gases. In a particular embodiment, nitrogen oxides in waste gases may be reacted with ammonia before the waste gases are discharged to the atmosphere.
A number of oxides of nitrogen are known which are relatively stable at ambient conditions and two of these, nitric oxide (NO) and nitrogen dioxide (NO.sub.2) are principal contributors to atmospheric pollution. In this specification, they are referred to generically as NO.sub.x for convenience. They are usually formed in internal combustion engines as well as in furnaces, boilers, and incinerators in which high temperature combustion processes occur. Although the concentrations of NO.sub.x in the exhaust gases of these combustion processes are relatively low, the aggregate amounts discharged into the atmosphere may be sufficient to cause significant pollution problems. Reduction of NO.sub.x emissions is therefore an objective in the control of industrial and automotive pollution.
Various reducing agents such as ammonia, carbon monoxide, hydrogen, and hydrocarbons may be used to convert NO.sub.x to molecular nitrogen (N.sub.2) in the presence of suitable catalysts. A number of metals and metal oxides are known to be catalytically active for the reaction, including platinum, rhodium, vanadium oxide, tungsten, and titanium oxide. See Bosch, H., et al., "Catalytic Reduction of Nitrogen Oxides--A Review of the Fundamentals of Technology," Catalysis Today, vol. 2, no. 4, 369-531 (1988). The metal component is conventionally supported on a porous carrier such as silica, alumina, zirconia, or a zeolite such as ZSM-5. See Groeneveld, M. J., et al., "Preparation, Characterization and Testing of New V/Ti/SiO.sub.2 Catalysts for Denoxing and Evaluation of Shell Catalyst S-995," Catalysis: Theory to Practice, Proceedings of the 9th International Congress on Catalysis, vol. 4, 1743-1749 (1988). A number of NO.sub.x reduction processes are referred to in U.S. Pat. No. 4,929,586 (Hegedus). U.S. Pat. No. 4,778,665 (Krishnamurthy) discloses a selective catalytic reduction process using a catalyst comprising an intermediate pore size zeolite.